How young animals interact with their siblings during their first months of life determines their social skills later on. This is shown by experimental research on the tropical freshwater fish Neolamprologus pulcher, published in PNAS. Not only does the number of siblings matter, but whether they can interact freely with one another does too.
For many animals, siblings are a key component of their social environment during early life. Previous research has shown that the early social environment is important, but it was not yet clear whether the number of siblings or the nature of their interactions is the decisive factor.
"The early social environment is often treated as a single, uniform factor," says lead author and behavioural ecologist Bruno Camargo dos Santos of Wageningen University & Research. "We wanted to experimentally disentangle what exactly makes the difference."
Three ways of growing up
In the experiment, young fish from the same clutch were randomly assigned, immediately after hatching, to one of three social conditions for their first three months of life.
One group grew up with 32 siblings, all able to interact directly with one another. A second group consisted of 8 siblings with equally unrestricted contact. In a third group, there were also 32 siblings, but divided across four compartments of 8 fish each. They could see and smell one another but could only interact directly within their own subgroup.
This design made it possible to distinguish the effect of group size from the effect of direct social interaction.
Group size and interaction shape social behaviour
During the first three months, young fish from the large group showed less aggressive and less submissive behaviour than fish from the other groups. At the same time, they showed more affiliative behaviours, spending more time together and following one another more often.
This indicates that growing up in a large group in which all siblings can interact freely results in a different social dynamic than growing up in a small group or in a situation where contact is restricted.
Effects remain visible
When the fish were five months old, the researchers tested their social competence - their ability to adjust their behaviour to the social situation. A larger, unfamiliar conspecific was introduced into their territory. In such a situation, the smaller fish has virtually no chance of winning.
"By displaying submissive behaviour quickly, the fish can limit further attacks and increase its chances of being accepted as a helper in the group - and therefore being allowed to remain in the territory," explains Bruno Camargo dos Santos.
For this species, acceptance is crucial: in their natural habitat, living in a group is essential for survival.
Fish from the large group displayed submissive behaviour more frequently when they received aggression than fish from small groups. They also showed less aggression after the larger fish had taken over the territory and were more often accepted. These behavioural patterns indicate higher social competence in fish raised in larger groups.
When social interactions were restricted in large groups, social competence decreased. These fish fell between the small and large groups in their responses. Only in avoidance behaviour did they resemble the large group: they withdrew less frequently than fish from small groups. This suggests that developing strong social competence requires both a high number of peers and opportunities to interact with them.
The foundations are laid early
The study shows that social skills do not emerge automatically. They are shaped during the first months of life by social interactions, especially among siblings. Growing up with many siblings alone is not sufficient; ample direct contact between them is also required.
"Social competence is a fundamental skill in social animals, including humans," says Bruno Camargo dos Santos. "Our results show that this competence is formed early in life through social interactions, especially with siblings. These early experiences influence how well individuals are accepted within a group later on."
